Mycobacterium smegmatis whmD and its homologue Mycobacterium tuberculosis whiB2 are functionally equivalent

Abstract

Mycobacterium smegmatis whmDis is an essential gene involved in cell division. This paper shows thatwhmDand its homologuewhiB2inMycobacterium tuberculosisare functionally equivalent. The genes are syntenous, and share significant homology in both their coding and non-coding DNA sequences. Transcription site mapping showed that the two genes possess near-identical promoter elements, and they displayed comparable promoter strengths in a reporter gene assay. The two proteins show near identity in their C-terminus, and polyclonal antiserum to WhmD specifically cross-reacts with a ∼15 kDa band inM. tuberculosislysates. Following overexpression of sense and anti-sense constructs in their cognate mycobacterial hosts,whiB2andwhmDtransformants displayed a small-colony phenotype, exhibited filamentation, and showed a reduction in viability. These observations reveal that the two proteins are functionally homologous and that their intracellular concentration is critical for septation in mycobacteria. Colonies ofM. tuberculosisoverexpressingwhiB2were spherical and glossy, suggesting a change in composition of the cell envelope. Filaments of the conditionally complementedM. smegmatis whmDmutant were non-acid-fast, also indicating changes in characteristics of surface lipids.M. smegmatistransformants carrying awhmD–gfpfusion showed a diffuse pattern of fluorescence, consistent with the putative role of WhmD as a regulator. These observations strongly suggest thatM. tuberculosis whiB2is an essential gene and its protein product in all likelihood regulates the expression of genes involved in the cell division cascade.